1. Field of the Invention
The present invention relates to a light source switching device for use in a projection system and particularly relates to a light source switching device that supplies light to a projection system by selecting different light sources according to a particular time sequence.
2. Descriptions of the Related Art
To reduce the power consumption and shrink the volume of the device, many projection systems have adopted light emitting diodes (LEDs) as the light source. Because the LEDs feature a short startup time, the use of the LEDs in a projection system may overcome the slow startup time of conventional high pressure mercury lamps. However, the use of a light source is restricted by the etendue conservation law which means that only the light falling within a limited light source area and a limited angular range is effective. Consequently, simply adding more LEDs will make no improvement to the brightness of an etendue limited projection system. Therefore, improving both the brightness and the efficiency of the light source using LEDs has long been a bottleneck to be broken through for the industry.
FIG. 1 illustrates a conventional projection system which drives the LEDs with intermittent pulses and improves the brightness by inputting a larger current. The projection system 1 comprises a first LED module 11a, a second LED module 11b, a mirror wheel 13, a motor (not shown), a digital micromirror device (DMD) 15, a prism 17 and a projection lens 19. Each of the LED modules 11a, 11b comprises red LED, green LED and blue LED (not shown) which alternately emit light to supply light. The mirror wheel 13 is disposed between the two LED modules 11a, 11b. 
The mirror wheel 13 comprises a plurality of alternately disposed reflective sectors and transmissive sectors which rotate about an axis thereof. When the first LED module ha emits light, the motor rotates one of the transmissive sectors of the mirror wheel 13 to a location corresponding to the light direction exiting from the first LED module 11a, so that the light travels through the transmissive sector and emits out in an output direction. On the other hand, when the second LED module 11b emits light while the first LED module 11a ceases emission, the motor rotates one of the reflective sectors of the mirror wheel 13 to a location corresponding to the light direction exiting from the second LED module 11b, so that the light is reflected and emits out in the output direction. In this way, the two LED modules 11a, 11b can supply the desired light in a rapid alternating order and results in an almost continuous light as perceived by naked eyes.
However, during operation, since the mirror wheel 13 consists of a plurality of alternately disposed reflective sectors and transmissive sectors, a number of border regions are inevitably formed therebetween. In case light from either LED modules impinges entirely or partly on such a border region, a portion of the light will not only be lost, but the instantaneous flux will also be degraded. To avoid the light loss, light from the LED modules must be controlled in such a way to keep away from the border regions as far as possible. However, since the two LED modules have their positions fixed beforehand, the only solution is to switch off a current operating LED module in advance when the border region of the mirror wheel 13 is approaching the current light beam and then switch on the opposite LED module immediately after the border region of the mirror wheel 13 passes the location corresponding to the light direction exiting from the opposite LED module. In other words, when the border regions are rotated to the locations corresponding to the light emitting direction of the LED modules, it shall be accompanied with an turned-off state of the LED modules as precisely as possible. Only after the border region passes the location corresponding to the light direction exiting from the LED modules can one of the LED modules be allowed to emit light.
However, the LED module provides a highly diffusive light beam that is different from a collective light beam provided by an ellipsoidal lamp or a parallel light beam provided by a parabola lamp. Hence, a light beam projected by an LED module will actually occupy a substantial area on the mirror wheel 13, rendering the aforesaid solution of switching the LED modules on and off impractical. What's more, when skipping the border regions, the numerous borders regions on the mirror wheel 13 will create a substantial unusable area on the mirror wheel 13, which will undoubtedly exacerbate the discontinuity in the light flux and degrade the utilization factor the mirror wheel 13 significantly.
On the other hand, the operations of the mirror wheel 13 and the motor have to be coordinated to switch between the LED modules. Because switching the LED modules is accomplished using electric signals, the mirror wheel 13 and the motor have a slow startup response and are liable to calibration errors and abrasion. As a result, it is difficult to accurately coordinate the switching speed and time intervals of the LED light sources. Moreover, this structure purely depends on the mirror wheels integrating the light beams. Every additional mirror wheel may significantly increase the dimension of the projection system, so improving the brightness is limited as restricted by the requirements of this industry on the volume.
In summary, the conventional LED light source structure for use in a projection system suffers from a discontinuous light flux, a slow light source switching speed, bulky volume and complex element configuration. Accordingly, it is highly desirable in the art to provide a light source that allows fast switching between the light sources, has a smaller size and is simple in structural configuration.